The movement of ions or molecules across a cell membrane (hereinafter referred to as “cell membrane ion transport”) is involved in formation and alteration of membrane potential which is important for cell functions. The cell membrane ion transport is made by a particular molecule (protein) included in the cell membrane. Ion pumps are molecules which allow the ions to move by expending some energy; and ion channels are molecules which open and close depending on the membrane potential, or upon binding of a particular molecule, thereby allowing the ion transport to be made corresponding to the concentration difference between the outside and the inside of the cell membrane.
The cell membrane ion transport is very important for cell functions, and if it is possible to grasp the cell membrane ion transport, it may be applicable in various ways. For example, ion channels are important as targets of drug discovery. In the past, evaluations of cell membrane ion transport have been usually made by a method using a patch-clamp technique (see, for example, Patent Document 1) or a method of measuring extracellular electric fields (see, for example, Patent Document 2). Methods of measuring intracellular ion concentrations by using fluorescence staining (see, for example, Patent Documents 3 and 4) have also been used.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-184207    Patent Document 2: Japanese Translation of PCT International Application Publication No. 2008-538287    Patent Document 3: Japanese Translation of PCT International Application Publication No. 2006-526389    Patent Document 4: Japanese Translation of PCT International Application Publication No. 2003-527113